fix(adapters): shared retry helper + run_log failure_class + enable RSS (issues #1 #2 #9)

- adapters/__init__.py: add http_get() unified retry (429/5xx only, max 2
  attempts, capped exp backoff) + AdapterHTTPError carrying failure_class;
  SourceAdapter.last_failure_class set on failure for pipeline capture.
- arxiv/github/huggingface/hackernews/reddit: route HTTP through http_get.
  Preserves GitHub 403 rate-limit retry and Reddit 403/429 fast-bail.
- schema.sql + pipeline.py: add run_log.failure_class column; rollup most-
  severe class across sources (5xx>4xx>429>error>zero_fetch>ok).
- pipeline.py: ENABLE RSS in ENABLED_SOURCES (was registered, disabled).
- RSS smoke test surfaced 3 broken feeds (anthropic 404, googleai 404,
  metaai 301) — left as-is, captured in feed_failures; URL fix is separate
  discovery task, not guessed.

Verified: full dry-run fetches all 6 sources; github live fetch OK;
Reddit 429 fast-bail preserved; no import/syntax errors.

adapters/arxiv.py

@@ -32,7 +32,7 @@ import xml.etree.ElementTree as ET
 from datetime import datetime, timedelta, timezone
 from html import unescape
 
-from adapters import SourceAdapter
+from adapters import SourceAdapter, http_get, AdapterHTTPError
 
 # arXiv API
 ARXIV_API = "http://export.arxiv.org/api/query"
@@ -151,14 +151,12 @@ class ArxivAdapter(SourceAdapter):
             f"&max_results={max_results}"
         )
 
-        req = urllib.request.Request(url, headers={"User-Agent": "ai-oracle/0.1"})
-
         try:
-            with urllib.request.urlopen(req, timeout=30) as resp:
-                xml_data = resp.read().decode("utf-8")
-            return self._parse_atom(xml_data)
-        except urllib.error.HTTPError as e:
-            print(f"    HTTP {e.code} for arXiv query")
+            raw = http_get(url, headers={"User-Agent": "ai-oracle/0.1"},
+                           timeout=30, max_retries=2, owner=self)
+            return self._parse_atom(raw.decode("utf-8"))
+        except AdapterHTTPError as e:
+            print(f"    {e.failure_class}: arXiv query ({e})")
             return []
         except Exception as e:
             print(f"    arXiv request error: {e}")